Ionic and non-ionic radiograhic contrast agents for use in combined x-ray and nuclear magnetic resonance diagnostics

ABSTRACT

The invention discloses the use of ionic and non-ionic radiographic contrast agents for combined X-ray and nuclear magnetic resonance (MRI) diagnostics.

The present invention discloses the use of radiographic contrast agentsfor the preparation of diagnostic formulations for the sequential orsimultaneous X ray and nuclear magnetic resonance (MRI) diagnostics.

X-ray contrast agents used in in vivo diagnostics are usuallycharacterized by high water solubility, low viscosity, low osmoticpressure, high contrast density, low toxicity and good tolerability.

The clinical use of non-ionic contrast agents has gradually replacedthat of ionic contrast agents which, on the other hand, are still beingused (see e.g. Renografin) in some diagnostic applications.

Examples of non-ionic contrastographic agents comprise Ioexol, Iomeprol,Iopentol, Iopromide, Ioversol, Ioxilan, Iodixanol and Iopamidol.

Magnetic Resonance Imaging diagnostic procedures mainly use paramagneticcompounds, preferably chelated complexes of bi- or trivalentparamagnetic metal ions with polyaminopolycarboxylic acids and/or theirderivatives or analogues.

Currently available MRI contrast agents comprise: Gd-DTPA, MAGNEVIST®;Gd-DOTA, DOTAREM®; Gd-HPDO3A PROHANCE®; Gd-DTPA-BMA, OMNISCAN®.

The contrast agents listed above are designed for a wholly general use.In fact, after intravenous administration, the MRI contrast agent isdistributed in the extracellular spaces in different parts of the bodyprior to being excreted. In this sense they are similar to iodinecompounds used in X ray medical diagnosis.

A recently proposed technique for MRI diagnostic is the magnetizationtransfer technique (see e.g. J. Chem. Phys. 39 (11), 2892-2901, 1963),in which the proton signal of a molecule, present in the medium or addedthereto, is suitably irradiated with a radiofrequency signal generatedby the apparatus magnetic field, and transferred through magnetizationto the water molecules surrounding the compound, i.e. the so-called“bulk water” of the medium.

The parameters affecting this process are related to different factorssuch as the nature of the chemical group involved in the protontransfer, the pH of the solution, the temperature of the medium and theintensity of the applied magnetic field.

U.S. Pat. No. 5,050,609 discloses the use of the saturation transfertechnique in magnetic resonance, which consists in the magnetizationtransfer in the presence of an irradiating field able to saturate theprotons involved in the exchange process, which procedure is used invitro and provides further information from the analyzed samples, e.g.biological tissues, polymeric compounds or samples of solid compounds ofgeologic interest.

Mag. Res. In Medicine, 44, 799-802, 2000, describes a procedure for thedetermination of the pH in solution using the magnetization signaltransfer technique in MRI, in the presence of 5-hydroxy trypthophan or5,6-dihydrouracil.

J. Mag. Res., 133, 36-45, 1998 and J. Mag. Resonance Imag., 12, 745-748,2000 describe in vivo and in vitro MRI imaging procedures involving thedetermination of the proton exchange between metabolites such as ureaand water.

Investigative Radiology, 23, S267-270, 1988, describes the effect ofsome compounds, such as arginine, glycine, Iopamidol, ornithine, serineand serinol, in MRI, in inducing a contrast increase by decreasing T2relaxation time, i.e. transversal relaxation time of water protons. Thedecrease of T2, which is experimentally observed, causes a decrease inthe intensity of the resulting imaging signal, and this is mainlyascribed to the chemical exchange between the mobile proton of themolecule and bulk water.

Mag. Res. Med. 35, 30-42, 1996 reports an interesting study on theeffect of proton exchange between amino acids and water surrounding thecompound in MRI, as a function of pH, temperature and presence of somecompounds acting as exchange catalysts in the medium.

Furthermore, both J. of Mag. Res., 143, 79-87, 2000 and WO00/66180disclose the use of some compounds in MRI with saturation transfertechniques of the proton signal.

The compounds used for this purpose belong to different chemicalclasses, consisting of sugars (e.g. mannitol, sorbitol, fructose,maltose, lactose and dextran), amino acids (e.g. L-Ala, L-Arg, L-Lys),nucleosides, purine and pyrimidine bases, barbituric acid, imidazolecompounds and other heterocyclic compounds.

A large number of compounds have been suggested as contrast agents forimaging of the vascular system and of the extra-vasal space, but theparenteral use thereof can involve unwanted side-effects which maketheir clinical use in vivo troublesome, although their enteral toxicityis not particularly high (see, for instance, Merck Index 12^(th) ed.:972. Barbital LD orally in mice: 600 mg/Kg; 973. Barbituric Acid LD50orally in male rats: >5000 mg/kg; 4475. Guanidine LD orally in rabbits:500 mg/kg RTECS Vol. 5 and. 1985-86: 82776. Thymidine LD50intraperitoneal mouse 2512 mg/kg; 60194. Pipecolinic Acid LD50intravenous mouse 2200 mg/kg; 9721. L-Arginine LD50 intravenous mouse2030 mg/kg; 41867. 2-Imidazolidinone LD50 intraperitoneal mouse 500mg/kg; 41826. 2-Imidazolidinetione LD50 intraperitoneal mouse 200mg/kg).

Nowadays, one of the most pressing requests by the medical classconcerns the availability of innovative contrast media for specificorgans or disorders, which usually are not effectively evidenced withthe known techniques.

X ray diagnostic contrast agents are an extremely versatile andinteresting class of compounds widely used in clinics thanks to theireffectiveness, low toxicity and safety of use in diagnosis concerningdifferent districts of the human body, such as in urography,angiography, ventriculography and myelography.

The use of the same contrast medium for both X ray radiology and MRIdiagnostics would remarkably widen the diagnostic potentialities of thecompound used and provide diagnostic results so far unexpected.

The availability of a single contrast agent providing diagnostic resultsthrough different techniques, which previously required the use ofdifferent contrast agents, is an important innovative aspect of thediagnostic technique, particularly in clinics, where diagnosticprocedures based on both the usual radiographic procedures, computerizedaxial tomography (CAT) and magnetic resonance imaging are nowadaysemployed.

In case of iodinated contrast agents, the use of a single compound for anumber of diagnostic procedures would be advantageous also since thetotal amount of the administered product is by far higher than that ofother medicaments.

By way of example, the dose of opacifying agent being injected can reachand even exceed 150 g and the combination of two diagnostic techniqueswould be cost-saving.

Therefore, the object of the present invention is the use of a iodinatedcontrast agent comprising at least one amido function for thepreparation of a diagnostic formulation to obtain in vitro or in vivoimages using magnetization transfer MRI techniques.

“Iodinated contrast agents comprising at least one amido function”herein means iodinated aromatic compounds having a triiodinated aromaticring bearing at the remaining positions straight or branched,functionally substituted organic residues or an organic compoundcomprising at least two triiodinated aromatic residues mutuallycovalently linked at one of the positions, either directly or through astraight or branched, functionally substituted organic residue, saidaromatic ring being further substituted at the remaining positions bystraight or branched, functionally substituted organic residues and inwhich the organic residues and the triiodinated aromatic ring are linkedby amido functions.

Examples of contrast agents for use according to the invention comprisethe compounds of general formula (I) and (II), (III) and (IV) reportedbelow, the corresponding isomers and stereoisomers, in particular theexo, endo regioisomers; the corresponding enantiomeric, racemic and mesoforms as well as the salts thereof with physiologically compatible basesor acids.

The method of the invention is based on the use of iodinated contrastagents having mobile protons able to exchange with bulk water, i.e. thesurrounding water molecules present in the medium and in the biologicaltissues.

Irradiation of the mobile proton(s) of the considered molecule with aradiofrequency field tuned on the resonance frequency of the concernedproton(s) induces saturation transfer to the bulk water signal caused bythe chemical exchange between the proton of the “exogen” molecule andwater. As a result, the saturation of the water signal appears as adecrease in the signal intensity in the MRI image obtained at the bodysite and/or on in vivo or in vitro samples, involved in the saturationtransfer.

The used procedure consists in the use of the contrast medium in vivo orin vitro and in performing the radiological and/or MRI diagnosticanalysis at different times and depending on the used technique,starting from one or the other diagnostic procedure.

The resulting MRI images are suitably acquired before, during and/orafter irradiation in the radiofrequency field of the proton thatexchanges with the water of the medium. The result from theexperimentation allows to compare images obtained with X rays and withMRI.

The sequence of the radiological and MRI techniques, which areconsidered complementary, can be reversed, depending on the body siteconcerned and the diagnostic analysis. In fact, it may be convenient tocarry out first MRI on the body angiographic district, then aradiological urographic investigation.

A further object of the present invention is the use of the disclosedMRI diagnostic technique alone, using the above defined iodinatedcompounds as contrast agents.

According to the invention the advantages deriving from the use of thisclass of compounds are that they have known, low in vivo and in vitrotoxicity, which makes them easy to use at different dosages, dependingon the product used and on the concerned body site. Other advantageousaspects of these compounds are their high water solubility and chemicalstability, and well-established pharmacokinetic (see e.g. RoFo Suppl.128, 220-223 (1989); Invest. Radio.,18, 368-374, 1983; Invest. Radio.,26, S156-S158, 1991) in terms of transport rate in circulation or inother body cavities, retention time in the organs being underexaminated, excretion and clearance.

The effect generated by saturation transfer of the protons of thecontrast agent molecule causes a decrease in the resulting MRI waterproton signal, which is the most important contribution to MagneticResonance images.

A first preferred group of iodinated contrast agents comprising amidofunctions are the compounds of formula (I)

in which:

-   -   A, D, E, which can be the same or different, are groups of        formula —CON(R)R₁, —COOH, —CONH₂ or —N(R)—COR₂ or CH₂N(R)—COR₂;    -   R is H or R₁, with the proviso that the substituent R is H in at        least one group of the compound;    -   R₁ is a straight or branched (C₁-C₆)alkyl residue, optionally        substituted with 1-5 hydroxy and/or alkoxy and/or hydroxy-alkoxy        groups, or with a NH—CO—R₁ or —CO—N(R)R₁ group, or R₁ is a        carbohydrate residue;    -   R₂ is a straight or branched (C₁-C₆)alkyl residue, optionally        substituted with 1-5 hydroxy and/or alkoxy and/or hydroxyalkoxy        groups and optionally interrupted by an oxo group.

A second preferred group of iodinated contrast agents comprising amidofunctions includes compounds of formula (II)

in which:

-   -   A, D and E are as defined above;    -   B and B′, which can be the same or different, are selected from        —CO—N(R)—, —N(R)—CO— or —N(COR₃)— groups, in which R is H or a        residue of a straight or branched (C₁-C₆)alkyl group, optionally        substituted with 1-5 hydroxy and/or alkoxy and/or hydroxyalkoxy        groups; R₃ is a (C₁-C₃)alkyl residue, optionally substituted        with 1-2 hydroxy or alkoxy or hydroxyalkoxy groups;    -   X is a covalent bond or a straight or branched (C₁-C₈)alkylene        chain, optionally substituted with 1-6 hydroxy and/or —CO—NHR        groups, and optionally interrupted by —O—, —S—, —N—, —N(R)—CO        groups;    -   in case both groups B and X are absent, the two aromatic        compounds are directly linked with a covalent bond with the        proviso that the substituent R is H in at least one group of the        compound.

Preferred compounds of general formula (I) are the compounds of generalformula (III) in which:

-   -   R₄, R₅, which can be the same or different, are H or a straight        or branched (C₁-C₃)alkyl group, which can optionally be        substituted with 1-2 hydroxy and/or alkoxy and/or hydroxyalkoxy        groups;    -   R₆ is a straight or branched (C₁-C₄)alkyl group containing one        or more hydroxy, alkoxy or acyloxy groups.

An example of particularly preferred compounds of general formula (I)and (III) are those compounds known under the names of Iopamidol andIopromide (see scheme 1).

A further preferred group of iodinated contrast agents comprising amidofunctions comprises compounds of formula (IV):

in which:

-   -   A′ is a OH or —NHR₁ group;    -   R₁ is a straight or branched (C₁-C₆)alkyl group, optionally        substituted with 1-5 hydroxy and/or alkoxy and/or hydroxyalkoxy        groups, or by a —NH—CO—R₁ or —CO—NHR₁ group, or R₁ is a        carbohydrate residue;    -   B″ and B′″, which can be the same or different, are H or R₁, as        defined above;    -   R₇ and R₈, which can be the same or different, are H, an acyl        —COR₁ group, an alkyl group, a mono or polyhydroxyalkyl group or        a carbohydrate residue;    -   with the proviso that at least one of the groups B″, B′″, R₇ or        R₈ is H.

A particularly preferred example of the compounds of formula (IV) isMetrizamide (see scheme 2 and U.S. Pat. No. 3,701,771, U.S. Pat. No.4,021,481), a water-soluble, soluble, non-ionic contrast agent havingmoderate toxicity and usually present as isomeric mixture.

A further preferred compound is diatrizoic acid (compound of formulaIV), Renografin® (see scheme 2 and e.g. Radiology 140: 507-511, 1981;Biochimica et Biophysica Acta. 756, 106-110, 1983, Elsevier BiomedicalPress and U.S. Pat. No. 4,192,859, U.S. Pat. No. 4,567,034 and U.S. Pat.No. 4,735,795) which is particularly suitable for the use according tothe present invention.

Examples of particularly preferred compounds of general formula (I) arethe compounds reported in Scheme 3 below. Scheme 3 Non- proprietary nameCAS [RN] A D E Metrizamide —CONHCH(CHO)(CHOH)₃CH₂OH —N(Me)Ac —NH—Ac[31112-62-6] Iopamidol —CONHCH(CH₂OH)₂ —CONHCH(CH₂OH)₂ —NHCOCH(OH)CH₃[60166-93-0] Iopromide —CONHCH(CH₂OH)₂ —CON(Me)CH₂CH(OH)CH₂OH—NHCOCH₂OMe [73334-07-3] Iogulamide —CONHCH₂CH(OH)CH₂OH—CONHCH₂CH(OH)CH₂OH —NHCOCO(CHOH)₃CH₂OH [75751-89-2] Iodamide —COOH—NHCOCH₃ —CH₂NHCOCH₃ [440-58-4] Ioglucol —CONHMe —NHCO(CHOH)₄CH₂OH—N(Ac)CH₂CH₂OH [63941-73-1] Ioglucomide —CONHMe —NHCO(CHOH)₄CH₂OH—NHCO(CHOH)₄CH₂OH [63941-74-2] Ioglunide —CONHCH₂CH₂OH —NHCO(CHOH)₄CH₂OH—N(Me)Ac [56562-79-9] Iobitridol —NHCOCH(CH₂OH)₂ —CON(Me)CH₂CH(OH)CH₂OH—CON(Me)CH₂CH(OH)CH₂OH [136949-58-1] Iocibidol —CON(Me)CH₂CH(OH)CH₂OH—CONHCH₂CH(OH)CH₂OH —CONH₂ [79211-34-0] MP-10007 —CONHCH₂CH₂OH—NHCOCO(CHOH)₃CH₂OH —NHCOCO(CHOH)₃CH₂OH [77111-65-0] Iotriside —CONH₂—CONHCH₂CH(OH)CH₂OH —CON(Me)CH₂CH(OH)CH₂OH [79211-34-0] Sodium —COONa—NHCOCH₃ —NHCOCH₃ diatrizoate [737-31-5]

Furthermore, particularly preferred are the compounds of formula (II)reported in the following Scheme 4. Scheme 4 Non- proprietary name CAS[RN] A D = E B-X-B Iofratol —CONHCH(CH₂OH)₂ —NHCOCH(OH)CH₃—CONHCH₂CH(OH)CH₂OH [141660-63-1] Iotasul —CON(Me)CH₂CH(OH)CH₂OH—CON(Me)CH₂CH(OH)CH₂OH —NHCOCH₂CH₂—S—CH₂CH₂CONH— [71767-13-0] (WO—CONHCH₂CH(OH)CH₂OH —NHCOCH₂OH —CONHCH₂CH(OH)CH₂NHCO— 9208691)[143200-04-8] (WO —CONHCH(CH₂OH)₂ —NHCOCH₂OH —CONHCH₂CH(OH)CH₂NHCO—9208691) [143199-77-3] (WO —CONHCH₂CH(OH)CH₂OH —NHCOCH₂OH—CONHCH₂C(CH₂OH)₂CH₂NHCO— 9208691) [143200-00-4]

Particularly preferred compounds for use according to the presentinvention comprise: Iopamidol, Iofratol, Iopromide, Metrizamide,Iogulamide, Ioglunide, Iobitridol, Iodamide, Sodium diatrizoate andother diatrizoic acid salts, and possible combinations thereof.

Other contrast agents for use according to the present invention aredescribed in the following patents, herein incorporated by reference:U.S. Pat. No. 4,364,921, U.S. Pat. No. 4,284,620, U.S. Pat. No.3,701,771, U.S. Pat. No. 4,001,323, U.S. Pat. No. 4,001,323, U.S. Pat.No. 4,250,113, U.S. Pat. No. 4,396,598, U.S. Pat. No. 4,192,859, U.S.Pat. No. 5,663,413, U.S. Pat. No. 4,239,747, U.S. Pat. No. 4,014,986,EP108 638, WO9208691, WO9515307, EP 33 426, U.S. Pat. No. 4,567,034,U.S. Pat. No. 4,735,795, U.S. Pat. No. 5,869,024, U.S. Pat. No.5,527,926, EP 431 838, EP 437 144.

The use of aqueous compositions of liposomes consisting of lipidmolecules mono-, bi- or multi-layers as carriers for the compounds usedas contrast agents is particularly preferred.

U.S. Pat. No. 4,192,859 discloses the preparation of liposomes made oflecithin and sterols, containing 20 to 60% by weight of contrast agentfor use in imaging of organs, and in particular of endothelial reticule,cardiovascular system and also for lymphographies. Compounds that can beused for this purpose (U.S. Pat. No. 5,445,810) comprise, for example,the following contrast agents:

Iopamidol, metrizamide, diatrizoic acid, sodium diatrizoate, megluminediatrizoate, acetrizoic acid and the soluble salts thereof, diprotizoicacid, Iodamide, Iodipamide sodium salt, Iodipamide meglumine salt,iodohippuric acid and the soluble salts thereof, iodometamic acid,iodopyracetiodo-2-pyridone-N-acetic acid, 3,5-diiodo-4-pyridone-N-aceticacid (Iodopiracet) and its diethyl ammonium salt, iothalamic acid,metrizoic acid and the salts thereof, ipanoic, iocetamic, iophenoxicacids and the soluble salts thereof, sodium tyropanoate, sodium hypodateand other similar iodinated compounds.

The preparation of liposomes containing opacifying contrast agents isdisclosed in the following patents, herein incorporated by reference:U.S. Pat. No. 4,192,859, FR-A-2561101, U.S. Pat. No. 4,567,034,GB-A-134869, GB-A-2135268, GB-A-2135647, GB-A-2156345, GB-A-2157283,EP-A-179660, U.S. Pat. No. 4,192,859, U.S. Pat. No. 4,744,989, U.S. Pat.No. 4,830,858, U.S. Pat. No. 5,393,530, U.S. Pat. No. 5,702,722, U.S.Pat. No. 5,895,661, U.S. Pat. No. 5,980,937, U.S. Pat. No 5,312,615,U.S. Pat. No. 5,445,810, U.S. Pat. No. 5,626,832.

Advantageous characteristics of the iodinated compounds described aboveare high water solubility, safety of use in vivo, pharmacologicalinertia, high chemical stability, low viscosity, low osmolarity andeffective magnetization transfer in the presence of the radiofrequencysignal applied at the absorption frequency of mobile protons.

Furthermore, said compounds have moderate in vivo toxicity and can beused at high dosages in diagnostic contrastographic applications.

Scheme 5 shows some data (Merck Index 12^(th) ed.) concerning toxicityof Iopamidol, Iopromide, Metrizamide and Sodium Diatrizoate.

Scheme 5

Iopamidol

-   -   4943. LD50 in mice, rats, rabbits, dogs (g/kg): 44.5; 28.2;        19.6; 34.7

Iopromide

-   -   4948. LD50 in mice, rats (g iodine/kg body weight): 16.5; 11.4        i.v.

Metrizamide

-   -   6077. LD50 i.v. in mice: 15 g/kg (Torsten); 18.6 g/kg        (Salvenson); 11.5 g/kg (Sovak); 17.3 g/kg (Aspelin).

Sodium diatrizoate

-   -   2975. LD50 i.v. in rats: 14.7 g/kg (Langecker).

Moreover, the chemical stability of said compounds is considered animportant feature which makes possible their treatment in autoclave athigh temperatures, during sterilization of pharmaceutical injectableforms.

As an example, pharmacokinetic and pharmacotoxicy of Iopamidol, amolecule characterized by three amido protons that exchange with waterprotons, have been the object of a number of clinical studies (see e.g.Radiologica Diagnostica, 7, 73-82, 1982; Diagnostic Radiology, 7, 83-86,1982; Drug Research, 40, 7, 1990; Clin. Pharmacokinet., 32, 180-193,1997).

Interestingly, Iopamidol (see Experimental Section) used in MRIsaturation transfer, allows to reduce even to 80% the signal from waterat a 2.1 T magnetic field.

The compounds of the invention can be used in various radiographicand/or MRI procedures, including those concerning intravascular imagingsuch as myelography, urography, angiography (e.g. cerebral andperipheral angiography), cardiography (e.g. coronary arteriographyand/or aortography), arthrography, for example of animal or human organssuch as heart, breast, brain, knee, liver and central nervous system.

An interesting application of the MRI technique of the present inventionis the use in angiographic diagnostics of blood vessels in organperfusion imaging.

Also interesting is the possibility of using the magnetization transferMRI technique, which allows differentiation between blood vessels andthe more vascularized tissues of an organ and those characteristic ofparenchyma.

An equation for rationalizing MRI magnetization transfer was firstformulated by Forsen and Hoffman in J. Chem. Physics, 39 (11), 2892-2901and is reported in the following:Ms/Mo≈[1/(1+KT)]  (1)wherein Ms is the value of the water proton signal during the saturationof the corresponding proton signal of the “exogen” molecule, Mo is thevalue of the signal without irradiation, i.e. in the presence ofirradiation with the opposite frequency to that irradiated during thesaturation phase, K is the exchange constant of mobile protons withwater and T is the spin-lattice relaxation time of water protons(J.Magn. Res., 133, 36, 1998).

Magnetization transfer is affected by some parameters, such as intensityof the applied magnetic field, intensity of the radiofrequency field,chemical group involved in the prototropic exchange with water, pH,temperature of the medium, and water and “exogen” compound contents inthe concerned tissue. An increase in the intensity of the radiofrequencyfield used for the saturation corresponds to an higher decrease of thebulk water signal. Both pH and temperature affect the proton exchangerate; in particular, an increase in temperature always induces anincrease in the exchange rate, while pH catalyzes the exchange as itdeviates from neutrality. Around pH 7, the exchange rate of the mobileprotons is minimum and it increases when the solution is acidified(pH<7) or alkalinized (pH>7).

The value of the applied magnetic field is very important in that, for agiven system, high B₀ values allows for higher exchange rates beforeincurring in a too large resonance broadening of exchangeable protons,which would make its efficient saturation with the irradiating field nolonger possible.

The magnetization transfer technique can be carried out coupled withdifferent kind of sequences, such as those known as “gradient or fieldecho” and “spin echo and fast spin echo”. However, the sequencescurrently used in imaging are “gradient or field echo” rather than “fastspin echo”.

The iodinated contrast agents are used at concentrations which varydepending on the compound used and the particular diagnostic use.

As a rule, the examination of a specific district of the human bodydetermines the amount and the corresponding concentration used.

The pharmaceutical formulation of the contrast agent, which is usuallyan aqueous solution, contains for example 15 g of compound per 100 mL ofsolution, corresponding to a iodine amount ranging from 50 to 500 mg permL. However, solutions containing lower amounts of the contrast agentcan also be used.

The amount of solution used in the investigations carried out accordingto the invention is usually comparable to that used in radiologicalanalysis and varies depending on the concerned body site: for example, 5to 15 mL in myelography, 3 to 5 mL in radiculography and 1 to 2 mL inventriculography. The solution or suspension containing the contrastagent can optionally be administered directly through the enteral route.For example, solutions or suspensions containing 1 to 100 mmoles ofcontrast agent, suspended in 1-2 l of physiologically compatiblesolution, can be used.

A further object of the present invention is an X ray and/or MRIprocedure comprising the administration of a pharmaceutical compositioncontaining a suitable amount of a compound or a mixture of compounds asdefined above, in particular of general formula (I), (II), (III) and(IV), for the imaging and the recording of the image of an organ of theindividual under observation.

The compounds of formula (I), (II), (III) and (IV) can be administeredthrough the oral or enteral route.

For the parenteral administration, they are preferably formulated assterile aqueous solutions or suspensions, with pH ranging from 6.0 to8.5.

Said aqueous solutions or suspensions can be administered inconcentrations ranging from 0.02 to 500 mM.

These formulations can be lyophilized and supplied as such, to bereconstituted just before the use. For the gastrointestinal use or forinjection to body cavities, these agents can be formulated as a solutionor suspension containing suitable additives in order to, for example,control viscosity, such as stabilizers, agents for controllingdissolution, anticoagulants, excipients used in the preparation offormulations and water soluble, physiologically compatible mineralsalts.

The following examples illustrate the invention in greater detail.

EXAMPLE 1

Effect of pH on 50 mM Iopamidol Solutions

The exchange rate between amido protons and water is affected by the pHof the solution. When measurements are carried out at 2.1T magneticfield and at a temperature of 39° C., the transfer effect is maximum atpH 7.5. This effect decreases as pH becomes both more acid and morebasic. When pH is below 6, or above 9, no effects are observed. In fact,outside this pH range from 6 to 9, the exchange is too fast for thepurposes of the invention.

The effect on the decrease of the water signal in the correspondingmagnetic resonance imaging, is anticipated by the measurements of R2(=/1T2) of water protons as a function of the solution pH. TABLE 1 R2values (=1/T2) of water protons in a 50 mM Iopamidol solution as afunction of pH at 2.1 and 9.34 T. Measurements were carried out with aJEOL EX-400 spectrometer at 25° C. using the CPMG sequence for thedetermination of T2. PH 3.3 4.7 5 5.6 6.3 6.8 7.3 8.0 8.55 9.24 10.5 R22.1T (s-1) 0.46 0.60 0.62 0.64 1.10 1.64 1.93 1.12 0.54 0.39 0.30 R29.34T (s-1) 30.8 32.3 31.3 32.0 30.0 32.8 40.1 41.7 35.3 32.4 34

EXAMPLE 2

Effect of the Temperature on a 200 mM Iopamidol Solution

A 200 mM Iopamidol solution at pH=6.74 was investigated at 2.1T magneticfield intensity (JEOL EX-90 spectrometer). After irradiating the amidoproton signal at 9.4 ppm, with attenuation of 300 dB, the area under thewater signal was measured. Considering 100 the area of the water signalwith irradiation at −9.4 ppm, a drastic reduction is observed followingirradiation of the amido protons. The effect increases as temperatureincrease so that: 15° C. 25° C. 39° C. % residual signal 39 30 24

EXAMPLE 3

Effect of Radiofrequency Field Intensity on a 200 Mm Iopamidol Solution.

A 200 Mm Iopamidol solution at pH=6.29 was investigated at 2.1T magneticfield intensity and at a temperature of 39° C. (JEOL EX-90spectrometer).

Considering 100 the area of the water signal with irradiation at −9.4ppm, its decrease (expressed as % residual signal) was evaluated in thepresence of a rf field tuned at 9.4 ppm (signal of the amido protons).

A 40% residual signal value is measured when the irradiating field hasan attenuation of 400 dB, which decreases to 15% when the attenuation is300 dB.

EXAMPLE 4

Evaluation of the Contrast Ability of 50 mM Iopamidol

Imaging of a 50 mM Iopamidol aqueous solution recorded with an MRIBruker Farmascan tomographer operating at 7.03 T. The sample is aplastic cylinder containing distilled water and a coaxial cone filledwith the Iopamidol aqueous solution at pH 7.4.

Measurements were carried out at a temperature of about 21° C. using aspin echo sequence coupled with the saturation transfer technique.

The results are shown in FIG. 1.

The main parameters used in said sequence are listed in the following:

-   -   matrix 256×128    -   FOV 3.5 cm    -   slice thickness 2 mm    -   repetition time 4 s    -   echo time 18.3 ms    -   MTC power level 9 μT    -   n° of MTC pulses 380 with 104 μs pulse length (pulse        shape=gauss).

The top left image was recorded applying the presaturation impulse atthe amido protons resonance frequency (i.e. at 1280 Hz from water), thetop right image was recorded applying the presaturation impulse at thefrequency opposite to that of amido protons relative to bulk water(−1280 Hz), the bottom left image was obtained from the differencebetween the two first images, whereas the bottom right image wasobtained without using the magnetization transfer technique.

EXAMPLE 5

Evaluation of the Contrast Ability of 25 mM Iopamidol

The procedures of Example 4 were followed, but using a 25 mM Iopamidolaqueous solution.

The results are shown in FIG. 2.

EXAMPLE 6

Evaluation of the Contrast Ability of 10 mM Iopamidol

The procedures of Example 4 were followed, but using a 10 mM Iopamidolaqueous solution.

The results are shown in FIG. 3.

1. The use of a iodinated contrast agent comprising at least one amido function for the preparation of a diagnostic formulation for obtaining in vivo or in vitro images with magnetization transfer MRI techniques, wherein the iodinated contrast agents comprising at least one amido function are iodinated aromatic compounds having a triiodinated aromatic ring bearing at the remaining positions straight or branched, functionally substituted organic residues or compounds comprising at least two triiodinated aromatic residues mutually covalently linked at one of the positions, either directly or through a straight or branched, functionally substituted organic residue, said aromatic ring being further substituted at the remaining positions by straight or branched, functionally substituted organic residues and in which the organic residues and the triiodinated aromatic ring are linked by amido functions.
 2. The use as claimed in claim 1 wherein magnetization transfer is coupled to “gradient or field echo” or “spin echo and fast spin echo” sequences.
 3. The use as claimed in claims 1 or 2 wherein the iodinated contrast agents are compounds of formula (I):

in which: A, D, E, which can be the same or different, are groups of formula —CON(R)R₁, —COOH, —CONH₂ or —N(R)—COR₂ or CH₂N(R)—COR₂; R is H or R₁, with the proviso that the substituent R is H in at least one group of the compound; R₁ is a straight or branched (C₁-C₆)alkyl residue, optionally substituted with 1-5 hydroxy and/or alkoxy and/or hydroxy-alkoxy groups, or with a NH—CO—R₁ or —CO—N(R)R₁ group, or R₁ is a carbohydrate residue; R₂ is a straight or branched (C₁-C₆)alkyl residue, optionally substituted with 1-5 hydroxy and/or alkoxy and/or hydroxyalkoxy groups and optionally interrupted by an oxo group.
 4. The use as claimed in any one of claims 1 to 3 wherein the iodinated contrast agents are compounds of formula (II):

in which: A, D and E are as defined above; B and B′, which can be the same or different, are selected from —CO—N(R)—, —N(R)—CO— or —N(COR₃)— groups, in which R is H or a residue of a straight or branched (C₁-C₆)alkyl group, optionally substituted with 1-5 hydroxy and/or alkoxy and/or hydroxyalkoxy groups; R₃ is a (C₁-C₃)alkyl residue, optionally substituted with 1-2 hydroxy or alkoxy or hydroxyalkoxy groups; X is a covalent bond or a straight or branched (C₁-C₈)alkylene chain, optionally substituted with 1-6 hydroxy and/or —CO—NHR groups, and optionally interrupted by —O—, —S—, —N—, —N(R)—CO groups; in case both groups B and X are absent, the two aromatic compounds are directly linked with a covalent bond with the proviso that the substituent R is H in at least one group of the compound.
 5. The use as claimed in any one of claims 1 to 3 wherein the iodinated contrast agents are compounds of formula (III):

in which R₄, R₅, which can be the same or different, are H or a straight or branched (C₁-C₃)alkyl group, which can optionally be substituted with 1-2 hydroxy and/or alkoxy and/or hydroxyalkoxy groups; R₆ is a straight or branched (C₁-C₄)alkyl group containing one or more hydroxy, alkoxy or acyloxy groups.
 6. The use as claimed in any one of claims 1 to 3 wherein the iodinated contrast agents are compounds of formula (IV):

in which: A′ is a OH or —NHR₁ group; R₁ is a straight or branched (C₁-C₆)alkyl group, optionally substituted with 1-5 hydroxy and/or alkoxy and/or hydroxyalkoxy groups, or by a —NH—CO—R₁ or —CO—NHR₁ group, or R₁ is a carbohydrate residue; B″ and B′″, which can be the same or different, are H or R₁, as defined above; R₇ and R₈, which can be the same or different, are H, an acyl —COR₁ group, an alkyl group, a mono or polyhydroxyalkyl group or a carbohydrate residue; with the proviso that at least one of the groups B″, B′″, R₇ or R₈ is H.
 7. The use as claimed in any one of claims 1 to 6 wherein the contrast agents are selected from Iopamidol, Iofratol, Ioprormide, Metrizamide, Iogulamide, loglunide, Iobitridol, Iodamide, Sodium diatrizoate and other diatrizoic acid salts, and combinations thereof.
 8. The use as claimed in any one of claims 1 to 7 wherein MRI images are acquired before, during and after irradiation in the used radiofrequency field.
 9. The use as claimed in claim 8 for radiographic and magnetic resonance imaging in myclographic, urographic, cerebral or peripheral angiographic, cardiographic, coronary arteriographic or aortograpbic, artrographic intravascular imaging investigations.
 10. The use as claimed in claim 9 wherein the radiological investigation is carried out in vivo or in vitro before or after the magnetization transfer magnetic resonance investigation.
 11. The use as claimed in any one of claims 1 to 10 wherein the diagnostic composition consists of a liposome suspension of the iodinated contrast agents together with suitable excipients.
 12. The use as claimed in any one of claims 1 to 11 wherein the saturation transfer to water depends on the pH of the solution or on the concerned body district.
 13. The use as claimed in any one of claims 1 to 12, wherein the saturation transfer to water depends on the temperature of the solution or on the concerned body district. 